FLOOD FORECASTING

Satellite flood image: http://www.crisp.nus.edu.sg/coverages/floods2007/index_p2.html

Why flood happen?

Occurs when the level of a body of water exceeds its natural or artificial confines

Water then submerges land in surrounding areas

Causes of Flooding Unbalance in hydrologic cycle Very high precipitation gives floods Very low precipitation gives

droughts Often combined effects:

Snow melt Inadequate drainage Water-saturated ground Dam failures High tides

River Flooding

Stage—height of a river Bankfull stage (or flood stage)—

when a river’s discharge increases to fill channel completely

Flood—water exceeds river’s banks

Floodplain

Area surrounding river influenced by flooding

Typically broad and flat, built of fine silt and mud from floodwaters

Usually very good agricultural land Eg.: Mississippi River floodplain

covers 80,000 square kilometers

Main features of a river valley

Upstream Flooding

Intense, infrequent storms of short duration

Cause flooding that is severe but local in extent

Called “upstream flooding” because effects of the storm runoff usually do not extend to the larger streams further downstream

Upstream floods are generally local, with short lag times

Flash Floods

Floods with exceptionally short lag time

Peak discharge reached only hours or minutes after storm has passed

Deadly

Downstream Flooding

Usually from storms that last a long time and extend over large area

Total discharge increases downstream as tributaries collect floodwaters

Downstream floods are regional in extent with longer lag times and higher peak discharges.

Examples of Flood Hazards

Primary Effects Water damage to household items Structural damage to buildings Destruction of roads, rail lines,

bridges, levees, boats, barges Historical sites destroyed Crop loss Cemeteries flooded, graves disrupted Loss of life

Examples of Flood Hazards

Secondary and Teritiary Impacts Destruction of farmlands Destructions of parklands and wildlife

habitat Health impacts

Disease related to pollution Injuries (back, electric shock, etc.) Fatigue Stress, depression

Examples of Flood Hazards

Disruption of transportation/electrical services

Gas leaks Lack of clean water

Secondary, Tertiary, continued

Impacts on crop prices; food shortages Job loss and worker displacement Economic impacts on industries

Construction (beneficial impact) Insurance (negative impact) Legal (beneficial impact) Farming (negative impact)

Misuse of government relief funds Changes in river channels Collapse of whole community structures

Flood Forecasting

To estimate the magnitude of flood peak, the following alternative methods are available:

Empirical Formula Rational Method Frequency Analysis

Empirical Formula Q = CAn

Where Q=Maximum flood discharge A=Catchment Area C=Constant that depend on

catchment & precipitation n=Index

Rarely used

Rational Method Q = C i A Where

Q=peak discharge (m3/s) C=coefficeint of runoff i = mean intensity of precipitation

(mm/hr) for duration equal to tc A=drainage area,km2

To compute Q, requires tc,i and C

Rational Method

For small size (<50 km2) catchments

This not cover what is MSMA (Manual Saliran Mesra Alam Malaysia)/Urban Stormwater Management Manual.

Rational Method

Runoff Coefficient

Runoff Coefficient Coefficient that represents the

fraction of runoff to rainfall Depends on type of surface When a drainage area has distinct

parts with different coefficients… Use weighted average

A

AC .. AC AC C

i

nn2211

Time of concentration, tc

For small drainage basin, tc=tp

For other catchment, use Kirpich Equation (1940)tc=0.01947 L0.77S-0.385

tc in min L= maximum length of travel time in m S= slope catchment = ∆H/L ∆H = difference of elevation between the most

remote point on the catchment and the outlet

Time of concentration, tc Sometimes its written as tc= 0.01947 K1

0.77

Where K1= H)/(L3

Rainfall Intensity, i

Corresponding to a duration tc and the desired probability of exceedence P

Return period, T=1/P Found from rainfall intensity—

duration-frequency (IDF) curve

Rainfall Intensity, i

Average intensity for a selected frequency and duration

Based on “design” event (i.e. 50-year storm) Overdesign is costly (what else?) Underdesign may be inadequate

Duration

Rainfall Intensity, i

Based on values of tc and T tc = time of concentration T = recurrence interval or design

frequency As a minimum equal to the time of

concentration, tc, (mm/hr)

Recurrence Interval (Design Event)

2-year interval -- Design of intakes and spread of water on pavement for primary highways and city streets

10-year interval -- Design of intakes and spread of water on pavement for freeways and interstate highways

50 - year -- Design of subways (underpasses) and sag vertical curves where storm sewer pipe is the only outlet

100 – year interval -- Major storm check on all projects

Time of Concentration (tc)

Time for water to flow from hydraulically most distant point on the watershed to the point of interest

Assumes peak runoff occurs when I lasts as long or longer than tc

Time of Concentration (tc)

Depends on: Size and shape of drainage area Type of surface Slope of drainage area Rainfall intensity Whether flow is entirely overland or

whether some is channelized

Example 1:Rational Method

An urban catchment has an area of 85 ha. The slope of the catchment is 0.006 and the maximum length of travel of water is 950m. The maximum depth of rainfall with a 25-year return period is as below

Duration (min)

5 10 20 30 40 60

Depth of rainfall (mm)

17 26 40 50 57 62

If a culvert for drainage at the outlet of this area is to be designed for a return period of 25years, estimate the required peak-flow rate, by assuming runoff coefficient is 0.3

Example 2:Rational Method If the urban area of example 1, the land

use of the area and the corresponding runoff coefficients are as given below, calculate the equivalent runoff coefficient.

Land Use Area (ha) Runoff coefficient

Roads 8 0.70

Lawn 17 0.10

Residential Area 50 0.30

Industrial Area 10 0.80